The present invention relates to froth flotation processes for recovery of metal values from base metal sulfide ores. More particularly, it relates to new and improved sulfide collectors comprising certain hydrocarboxycarbonyl thionocarbamate compounds which exhibit excellent metallurgical performance over a broad range of pH values.
Froth flotation is one of the most widely used processes for beneficiating ores containing valuable minerals. It is especially used for separating finely ground valuable minerals from their associated gangue or for separating valuable minerals from one another. The process is based on the affinity of suitable prepared mineral surfaces for air bubbles. In froth flotation, a froth or a foam is formed by introducing air into an agitated pulp of the finely ground ore in water containing a frothing or foaming agent. A chief advantage of separation by froth flotation is that it is a relatively efficient operation at a substantially lower cost than many other processes.
Current theory and practice state that the success of a sulfide flotation process depends to a great degree on the reagent(s) called collector(s) that impart(s) selective hydrophobicity to the value sulfide mineral that has to be separated from other minerals. Thus, the flotation separation of one mineral species from another depends upon the relative wettability of mineral surfaces by water. Typically, the surface free energy is purportedly lowered by the adsorption of heteropolar collectors. The hydrophobic coating thus provided acts in this explanation as a bridge so that the mineral particles may be attached to an air bubble. The practice of this invention is not, however, limited by this or other theories of flotation.
In addition to the collector, several other reagents are also necessary. Among these, the frothing agents are used to provide a stable flotation froth, persistent enough to facilitate the mineral separation, but not so persistent that it cannot be broken down to allow subsequent processing. The most commonly used frothing agents are pine oil, creosote and cresylic acid and alcohols such as 4-methyl-2-pentanol, polypropylene glycols and ethers, etc.
Moreover, certain other important reagents, such as the modifiers, are also largely responsible for the success of flotation separation of sulfide minerals. Modifiers include all reagents whose principal function is neither collecting nor frothing, but one of modifying the surface of a mineral so that a collector either adsorbs to it or does not. Modifying agents can thus be considered as depressants, activators, pH regulators, dispersants, deactivators, etc. Often, a modifier may perform several functions simultaneously. Current theory and practice of sulfide flotation again state that the effectiveness of all classes of flotation agents depends to a large extent on the degree of alkalinity or acidity of the ore pulp. As a result, modifiers that regulate the pH are of great importance. The most commonly used pH regulators are lime, soda ash and, to a lesser extent, caustic soda. In sulfide flotation, however, lime is by far the most extensively used. In copper sulfide flotation, which dominates the sulfide flotation industry, for example, lime is used to maintain pH values over 10.5 and more usually above 11.0 and often as high as 12 or 12.5. In prior art sulfide flotation processes, preadjustment of the pH of the pulp slurry to 11.0 and above is necessary, not only to depress the notorious gangue sulfide minerals of iron, such as pyrite and pyrrhotite, but also to improve the performance of a majority of the conventional sulfide collectors, such as xanthates, dithiophosphates, trithiocarbonates and thionocarbamates. The costs associated with adding lime are becoming quite high and plant operators are interested in flotation processes which require little or no lime addition, i.e., flotation processes which are effectively conducted at slightly alkaline, neutral or even at acid pH values. Neutral and acid circuit flotation processes are particularly desired because pulp slurries may be easily acidified by the addition of sulfuric acid, and sulfuric acid is obtained in many plants as a by-product of the smelters. Therefore, flotation processes which do not require preadjustment of pH or which provide for pH preadjustment to neutral or acid pH values using less expensive sulfuric acid are preferable to current flotation processes because current processes require pH preadjustment to highly alkaline values of at least about 11.0 using lime which is more costly.
To better illustrate the current problems, in 1980, the amount of lime used by the U.S. copper and molybdenum mining industry was close to 550 million pounds. For this industry lime accounted for almost 92.5% by weight of the total quantity of reagents used, and the dollar value of the lime used was about 51.4% of the total reagent costs for the industry, which amounted to over 28 million dollars.
As has been mentioned above, lime consumption in individual plants may vary anywhere from about one lb. of lime/metric ton of ore processed up to as high as 20 lbs. of lime/metric ton of ore. In certain geographical locations, such as South America, lime is a scarce commodity and the costs of transporting and/or importing lime have risen considerably in recent years. Still another problem with prior art highly alkaline processes is that the addition of large quantities of lime to achieve sufficiently high pH causes scale formation on plant and flotation equipment, thereby necessitating frequent and costly plant shutdowns for cleaning.
It is apparent, therefore, that there is a strong desire to reduce or eliminate the need for adding lime to sulfide flotation processes to provide substantial savings in reagents costs. In addition, reducing or eliminating lime in sulfide ore processing may provide other advantages by facilitating the operation and practice of unit operations other than flotation, such as slurry handling.
In the past, xanthates and dithiophosphates have been employed as sulfide collectors in froth flotation of base metal sulfide ores. A major problem with these conventional sulfide collectors is that at pH's below 11.0, poor rejection of pyrite or pyrrhotite is obtained. In addition, with decreasing pH the collecting power of these sulfide collectors also decreases, rendering them unsuitable for flotation in mildly alkaline, neutral or acid environments. This decrease in collecting power with decreasing pH, e.g., below about 11.0, requires that the collector dosage be increased many fold, rendering it generally economically unattractive. There are many factors which may account for the lowering of collector activity with decreasing pH. A collector may interact differently with different sulfide minerals at a given pH. On the other hand, poor solution stability at low pH, such as that exhibited by xanthates and trithiocarbonates may very well explain the observed weak collector behavior.
Efforts to overcome the above deficiencies led to the development of neutral derivatives of xanthates such as alkyl xanthogen alkyl formates generally illustrated by the formula: ##STR2## The alkyl xanthogen alkyl formates are disclosed as sulfide collectors in U.S. Pat. No. 2,412,500. Other structural modifications of the general structure were disclosed later. In U.S. Pat. No. 2,608,572 for example, the alkyl formate substituents contain unsaturated groups. In U.S. Pat. No. 2,608,573, the alkyl formate substituents described contain halogen, nitrile and nitro groups. Bis alkyl xanthogen formates are described as sulfide collectors in U.S. Pat. No. 2,602,814. These modified structures have not found as much commercial application as the unaltered structures. For example, an alkyl xanthogen alkyl formate is currently commercially available under the trade name MINEREC.RTM.A from the Minerec Corporation. MINEREC.RTM.A, an ethyl xanthogen ethyl formate, as well as its higher homologs, still leave a lot to be desired at pH below 11.0 in terms of collecting power and pyrite rejection, as is more particularly described hereinafter.
Another class of sulfide collectors which have obtained some degree of commercial success in froth flotation are oily sulfide collectors comprising dialkylthionocarbamate or diurethane compounds having the general formula: ##STR3## Several disadvantages are associated with the preparation and use of these compounds. In U.S. Pat. No. 2,691,635, a process for making dialkylthionocarbamates is disclosed. The three steps of the reaction sequence described are cumbersome and the final by-product is ethyl mercaptan, and air pollutant which is costly to treat. In U.S. Pat. No. 3,907,854 an improved process for making dialkylthionocarbamate is described. Although good yields and high purity are claimed as the novel features of the process, it is noteworthy that a side product of the reaction is sodium hydrosulfide, also a pollutant which requires special treatment for disposal. In U.S. Pat. No. 3,590,998 a thionocarbamate sulfide collector structure in which the N-alkyl substituent is joined by alkoxycarbonyl groups is disclosed. The preparation process described therein requires the use of expensive amino acid esters of the displacement reaction of the thio esters of xanthates. The by-products of this process are either methyl mercaptan or sodium thioglycolate. In addition, this type of structurally modified thionocarbamate has enjoyed very little commercial success. As will become apparent from the disclosure of this invention below, dialkylthionocarbamates are weak collectors as the pH drops below certain values.
Accordingly, it is an object of the present invention to provide a new and improved sulfide collector and flotation process for the beneficiation of sulfide minerals employing froth flotation methods which does not require any pre-adjustment of pH to highly alkaline values.
It is another object of the present invention to provide a new and improved sulfide collector and froth flotation process for the beneficiation of sulfide minerals which provides selective recovery of sulfide metal values with selective rejection of pyrite, pyrrhotite and other gangue sulfides.
It is a further object of the present invention to provide a new and improved sulfide collector and flotation process for the beneficiation of sulfide minerals using froth flotation methods which employs a novel class of sulfide collector reagents which may be prepared and used without the formation of harmful by-products or environmental pollutants.
It is another object of the present invention to provide a flotation process for the beneficiation of sulfide ores at pH values of 10.0 or below using certain novel collectors containing novel donor atom combinations designed specifically for low pH flotation.
It is still another object of the present invention to provide a new and improved process for selective flotation of value sulfides in acid circuits, wherein inexpensive sulfuric acid is used to control the pH.